1. Field of the Invention
This invention relates to a method of estimating heat resistivity for a spark plug and a device of estimating the same which makes use of ignition manners represented by an ignition type internal combustion engine.
2. Description of Prior Art
In general, it is necessary to select some heat value for a spark plug which enables to normally operate an internal combustion engine under every condition, since a heat range depends on the heat value in which the spark plug works normally in the ignition type internal combustion engine. A method of detecting of preignition has been utilized to recognize an allowance of the heat range up to a upper limit as a way of estimating a heat resistivity for the spark plug.
The so-called preignition is one of the ignition manners represented by the ignition type internal combustion engine in which an air-fuel mixture gas is ignited prematurely by a hot spot (insulator surface in general) of the spark plug before a discharge spark occurs across the electrodes of the spark plug. This type of the preignition is uncontrollable. The method of detecting of preignition means to check a heat resistivity for the spark plug as referred to a method of estimating heat resistivity. In this method, the preignition is generally induced by advancing the ignition timing to increase a heat load imposed on the spark plug, and thus rises its temperature so as to measure a thermal allowance of the spark plug. The advantage of this method is to directly recognize the allowance of heat resistivity for the spark plug to be measured.
However, when the ignition timing is advanced so far as to cause the preignition, this induces a rapid temperature rise within a combustion chamber of the internal combustion engine, and the repetitive occurrences of the preignition poses a problem to do damage seriously on the internal combustion engine. In such an internal combustion engine as the ignition timing is determined in the proximity of knocking limit by a knocking control system or the like, an advanced allowance of the ignition timing is very restricted so that the allowance of the heat resistivity is not directly recognized since the heat value is assumed by using lower heat rating of the spark plug than presupposedly used.
Aside from the preignition method, a postignition method has been utilized which estimates a heat compatibility without changing the ignition timing. The postignition method is one of the ignition manners other than the preignition in which the air-fuel mixture gas is ignited after the discharge spark occurs across the electrodes of the spark plug. This manner of the postignition is further divided into two types. One is controlled ignition as caused by the normal ignition spark, and another is ignition uncontrollable by the ignition spark (self-ignition). The latter type of the uncontrollable ignition is generally caused from the heat surface in which deposits are placed on the spark plug and the combustion chamber of the internal combustion engine.
The postignition method is generally referred to the latter type of the uncontrollable ignition which utilizes the ignition from the hot spot of the spark plug so as to check the heat resistivity for the spark plug. In the postignition method, when the ignition spark is suppressed at a normal advancement of ignition at regular intervals on the assumption that the ignition does not occur due to the deposits on the hot spot of the spark plug or the like, a rate of the postignition occurrence is determined on the basis of the occurrences of the postignition so as to measure a degree of the thermal allowance of the spark plug. The advantage of the postignition method is to estimate the heat value without advancing the ignition, thus obviating the serious damage on the internal combustion engine as done in the preignition method.
However, in this type of postignition method, the thermal allowance of the spark plug is recognized to examine whether or not the heat value of the spark plug is appropriate only in the rate of the postignition occurrence.
TABLE 1 ______________________________________ postignition occurrence ignition timing in which rate at ignition preignition actually timing of 20.degree. BTDC occurs (heat value) ______________________________________ spark plug A 33.8% 31.degree. BTDC spark plug B 70.2% 31.degree. BTDC ______________________________________
In the spark plugs A, B of different internal structure, but same heat value as shown in TABLE 1, the rate of the postignition occurrence differs between the spark plugs A, B when the ignition timing is measured in terms of 20.degree. BTDC (Before Top of Dead Center) because the former falls on 33.8%, and the latter 70.2%. By way of illustration, when the heat value of the spark plug is appropriate below 50% rate of the postignition occurrence, the spark plug B is mistaken for having a lower heat value although the spark plugs A, B are of the same heat value.
Therefore, it is one of the objects of the invention to provide a method of estimating heat resistivity for a spark plug and a device of estimating the same which is capable of measuring a heat resistivity of a spark plug without doing serious damage on an internal combustion engine, and without changing an ignition timing in which a discharge spark occurs across electrodes of the spark plug.
It is also one of the objects of the invention to provide a method of estimating heat resistivity for a spark plug and a device of estimating the same which is capable of anticipating an ignition timing in which a preignition occurs from the spark plug.
Further, it is one of the objects of the invention to provide a method of estimating heat resistivity for a spark plug and a device of estimating the same which is capable of correctly selecting a heat value compatible with the spark plug by estimating its heat resistivity.